The present invention relates to optical loss measurement which are generally carried out in order to determine the optical characteristics of an optical component
A typical optical loss measurement is the return loss measurement. The return loss (RL) of an optical component is generally defined as the ratio of the reflected optical power Pback to the incident optical power Pin, in units of dBopt. Therefore, the return loss is usually a positive number with:
RL=xe2x88x9210 log (Pback/Pin) in [dBopt]xe2x80x83xe2x80x83(eq. 1).
FIG. 1a shows an arrangement for determining the return loss by means of a fiber optical RL-meter 10. The RL-meter 10 comprises a source 20 (e.g. a laser source), a receiver 30 (e.g. an optical power meter), a fiber optical coupler 40, and a connection 50 (e.g. a front panel connector) of the RL-meter 10 to a device under test (DUT) 60.
The fiber optical coupler 40 is normally embodied by a fused fiber coupler as depicted in FIG. 3a. The optical coupler 40 comprises a first fiber with a first end 41 and a second end 42 and a second fiber with a third end 43 and a forth end 44. The first and second fibers are coupled in a way that a signal coming from one side (e.g. end 41) is coupled to the ends (e.g. ends 42 and 43) of the other side. The optical coupler 40 provides a strict directivity, so that the incident beam at one side is split up (e.g. in equal amounts) and provided at the ends of the opposing side, whereas only a small amount (e.g. about 10xe2x88x926 . . . xe2x88x927) of the incident beam will be reflected to the other end of the side of the incident beam.
When an optical power Ps is provided at the end 41, an optical power M can be measured at the end 42 which substantially corresponds to the optical power Ps, with M=t1xc2x7Ps. When an object with a given reflectivity R is coupled to the end 44, a returning optical power P can be measured at the end 43, with:
P=c1xc2x7Mxc2x7R+c2xc2x7Mxe2x80x83xe2x80x83(eq. 2),
whereby c1 and c2 represent general factors depending on the characteristics of the fiber coupler 40.
Before measuring the return loss of the DUT 60, a calibration of the RL-meter 10 needs to be done, e.g. as described by Christian Hentschel, xe2x80x9cFiber Optics Handbookxe2x80x9d, third Edition, March 1989, Hewlett-Packard, on page 188. As shown in FIG. 1b thereof, a cable 70 is connected to the connector 50. The return loss calibration and measuring procedure consists of three steps. In a first step, a calibration setup is performed with a connector 80 of the cable 70 open. A power meter of the receiver 30 reads a power P1. In a second step, the connector 80 is immersed in oil in order to avoid reflections from the end of the fiber. A measurement of the unwanted reflections from the pair connector 50 and connector 90 of the cable 70 is performed. The power meter now reads P2. The calibration can then be done based on the measured power values P1 and P2. Finally, the DUT 60 is connected to the connector 80 and measurements of the DUT 60 can be performed in a third step (see FIG. 1c).
More details about return loss measurements are also given by Dennis Derickson, Fiber Optic Test and Measurement, ISBN 0-13-534330-5, 1989, e.g. P. 387ff and P. 461ff.
Another typical optical loss measurement is the insertion loss measurement. The insertion loss (IL) of an optical component is generally defined as the ratio of the transmitted optical power Pout to the incident optical power Pin, in units of dBopt:
IL=xe2x88x9210 log (Pout/Pin) in [dBopt]xe2x80x83xe2x80x83(eq. 3)
FIG. 2a shows a typical measurement setup for insertion loss measurements using substantially the same measurement components as for the return loss measurement in FIG. 1. The source 20 can be coupled via the fiber coupler 40, or directly, to the connector 50 which again couples via the cable 70 to the DUT 60. Another end of the DUT 60 is coupled via a connector 100 to the receiver 30. Again, before measuring a calibration of the measurement setup generally has to be performed.
FIG. 2b shows a calibration step for the insertion loss measurement. The connectors 80 and 100 are directly coupled together, and the receiver 30 measures the output power Pout. For measurement purposes, the DUT 60 is inserted between the connectors 80 and 100, as shown in FIG. 2a. More details about typical insertion loss measurements are given by Christian Hentschel, xe2x80x9cFiber Optics Handbookxe2x80x9d, third Edition, March 1989, Hewlett-Packard, on page 188, or in Dennis Derickson, Fiber Optic Test and Measurement, ISBN 0-13-534330-5, 1989, P. 21-22, P. 339-382 and P. 454-457.
In most applications, the calibration of the return loss measurement is performed using a specific reference cable 70R as the cable 70. The reference cable 70R normally provides a defined return loss and a minimum insertion loss, e.g. due to minimized mechanical tolerances and excellent polishing, and allows a well defined calibration in a defined measurement environment. xe2x80x98Normalxe2x80x99 measurements of the DUT 60 are then carried out using an xe2x80x98ordinaryxe2x80x99 so-coled customer cable 70C as the cable 70. It is highly recommended to use different cables for calibration and measuring, since the connector 80 (of the reference cable 70R) might be degraded when frequently changing the DUTs 60
When using different cables 70 for calibration and measuring, however, the insertion loss of the different cables 70 might be different because of mechanical tolerances of fiber optic connectors. A change of x dB at a certain connection will result in a 2x dB measurement error in the return loss measurement and generally decrease its accuracy, since any loss change in the measurement setup will influence the detected power level at the receiver 30. For return loss measurements, the radiation goes twice through the connection of the RL-meter 10 to the attached cable 70, forward and reverse, thus influencing twice the loss change of the return loss measurement result.
Other inaccuracies of the return loss measurement might occur from a variation of the output power (e.g. a drift versus time or temperature) and/or of the optical spectrum of the source 20. This leads to a variation of the insertion and return loss of the components involved in the respective measuring setup, e.g. of the connector 50.
It is an object of the present invention to provide an improved loss calibration and/or measurement for optical components. The object is solved by the independent claims. Preferred embodiments are given by the dependent claims.
A first aspect of the invention concerns an improved fiber coupler as set out in claim 10 allowing to reduce an influence of reflection on the measuring results.
A second aspect of the invention concerns the calibration of a system for determining an optical loss of a device under test DUT as set out in claim 1, and the determination of a return loss of the DUT as set out in claim 3.
A third aspect of the invention concerns a further improved determination of the return loss of the DUT as set out in claims 5 and 7.
A fourth aspect of the invention concerns the determination of the insertion loss of the DUT as set out in claim 8.